INORGANIC SALTS. 39 



proper effect. Preparations stained with it may be 

 mounted in dilute glycerine. 1 



NITRO-PRUSSIATE OF SODIUM. 

 This is useful in its aqueous solution for the 

 detection of free sulphur. 2 It should be prepared 

 when required for use. The crystals need to be 

 kept from the air, from which they very readily 

 take up water. The preparation to be tested is 

 heated with potassic hydrate, after which treat- 

 ment the granules of sulphur unite to form larger 

 yellow masses, which are colored violet by the 

 nitro-prussiate. 



POTASSIC FERRO-CYANIDE (Yellow Prussiate). 



An aqueous solution precipitates ferric salts, 

 thus causing a blue color. This reaction has 

 been taken advantage of for the detection of fer- 

 ric hydrate in the cell-wall, e. g. in Crenothrix. 3 

 Cells, the brown color of whose walls causes a 

 suspicion of an encrusting compound of iron, are 

 treated with a mixture of hydrochloric acid and 

 the ferro-cyanide. If the beautiful color of Prus- 

 sian blue appears the suspicion caused by the 

 brown color is confirmed. 



1 W. Hassloch: New York Med. Journ., Nov. 1878, also Journal 

 Royal Microscop. Soc., 1879, Vol. II., p. 170. I have not tested this my- 

 self. 



2 Cohn: Untersuchungen liber Bacterien, II. Beitr. zur Biol. d. 

 Pfl., Bd. I., Heft 3, p. 175. 



8 Cohn; Ueber den Brunnenfaden. - Beitr. zur Biol. d. Pfl., Bd. II., 

 Heft i, p. 119. 



